The omnivore's other dilemma: Eating meat and the link between the gut's bacteria and heart disease

Eating meat and the link between the gut's bacteria and heart disease

We know that our gut is important to our health.

But a series of new findings, published by researchers from Cleveland Clinic (the parent organization of Cleveland Clinic Canada, where I work) represent a quantum leap forward in our understanding of just how the bacteria that naturally exist in our intestinal tract can contribute to our risk of disease. And if you’re an omnivore, the news isn’t great.

For years, we’ve known that vegetarian and vegan diets tend to be associated with a reduced risk of heart disease, perhaps because of the fibre, vitamins, minerals, or plant-derived polyphenols, which are particularly abundant in diets rich in fruits, vegetables, whole grains, and legumes. We also have a long line of evidence suggesting that animal fats, and red meat in particular, can be hard on our heart. But this new research, which has been published in the mega-journals Nature Medicine and The New England Journal of Medicine, has opened up an entirely new angle in the plant-versus-animal discussion.

A new study suggests saturated fats and cholesterol may not be the only reasons to limit consumption of red meat.

Scientists at the Cleveland Clinic, a nonprofit academic medical center in Ohio, have found a link between the organic bacteria compound carnitine, which is common in red meat, and an increased risk of heart disease.

According to researchers, carnitine is not a danger in and of itself, but once it is digested in the stomach after a person eats red meat, it is then metabolized in the liver and quickly transformed into an unhealthy compound known scientifically as Trimethylamine N-oxide (TMAO), a known trigger of heart disease.

The research team, led by Dr. Stanley Hazen, MD, PhD, who, among his titles, is section head of Preventive Cardiology & Rehabilitation in the Miller Family Heart and Vascular Institute, began their studies with a question: why do some people have heart attacks (or strokes, or other cardiovascular events) that simply cannot be predicted? Yes, cholesterol, blood pressure, and markers of inflammation paint some of the picture, and genetics add another layer — though, as Hazen noted in an interview, our genes only seem to predict about 10% of our overall risk — but there is still a gap between what we predict will happen and what actually happens that remains unexplained.

Enter the gut bacteria. In a series of elegant studies, Hazen’s team began by analyzing the plasma (blood) of patients who undergone screening for heart health. After following the patients for three years, they looked specifically at those who had experienced a heart attack or stroke, or who had died, and compared them with age- and gender-matched controls. Without knowing what they were looking for, the research team found that three compounds were consistently present in the blood of those who had had the cardiac events: trimethylamine N-oxide (TMAO), betaine, and choline. As luck would have it, all three are derived from the same metabolic pathway: the breakdown of phosphatidyl choline, also known as lecithin, a nutrient predominantly found in eggs, milk, organ meats, red meat, poultry, shellfish, and fish — in other words, animal products.

Choline, which is converted to TMAO in the digestive tract, accelerated atherosclerosis (hardening of the arteries) in animals

Since the researchers had no idea what compounds they were looking for, they actually had to do some research of their own on TMAO, and soon found studies demonstrating that choline, which is converted to TMAO in the digestive tract, accelerated atherosclerosis (hardening of the arteries) in animals.

Next, Hazen’s group looked at the relationship between TMAO and heart health in a large human population, and sure enough, they found a very clear association with high circulating levels of TMAO and heart disease risk. But that still doesn’t prove cause-and-effect. So, to come one step closer, they gave a group of subjects two eggs — one of the richest sources of lecithin, the nutrient that breaks down into choline, which is then fermented to TMAO — and sure enough, TMAO levels climbed shortly after.

Finally, to confirm the relationship between the intestinal bacteria and TMAO, they gave the subjects antibiotics to wipe out the bacteria in their digestive system, and TMAO levels crashed.

After feeding subjects a carnitine-rich food – steak — they saw a similar spike in TMAO levels as they had with eggs

In a separate set of experiments, the research team also examined the effect of carnitine, a prominent compound in animal foods that is chemically related to lecithin, on TMAO and heart disease risk. After feeding subjects a carnitine-rich food – steak — they saw a similar spike in TMAO levels as they had with eggs. They also found that TMAO encourages more cholesterol to be deposited into the arteries, while simultaneously reducing the body’s ability to eliminate it. As with the studies on choline, the research team also measured the capacity of carnitine levels in the blood to predict heart disease in a pool of over 2,500 subjects, and found that, as Hazen put it, “carnitine is as good or better as anything we are currently using to measure for atherosclerosis. But really, when we isolated for just TMAO, it was the real culprit.”

And here’s another twist: If you give meat to vegans (who exclude all animal products from their diet), you don’t see an increase in TMAO levels. Why? Because since their body isn’t exposed to animal foods, they don’t have the bacteria needed to convert choline into TMAO; if they became omnivores, however, levels of the bacteria would start to climb. “We are like a walking petri dish,” noted Hazen, “the more we eat of a particular food, the more we shift our gut flora in favour of those foods.”

‘The more we eat of a particular food, the more we shift our gut flora in favour of those foods’

And eating more animal products, it would seem, can push our body to produce more TMAO, with potentially serious consequences.

If these findings leave you feeling bewildered, you’re not alone. After all, eggs have largely been on the nutritional upswing lately, and carnitine is a popular in sports supplements and energy drinks. So now we are to believe that they are triggering heart attacks? They might, but remember that this research is still very new. There may, for example, be other factors that can help us to control TMAO levels, much like oat bran helps to reduce cholesterol, that simply haven’t been discovered yet. In the meantime, it
makes sense to follow existing recommendations to emphasize plant foods in our diet, while keeping meat and other animal foods, which tend to be rich in both carnitine and lecithin, moderate (remember that your body cannot use any more than about 30 grams of protein, equal to serving of meat about the size of the palm of your hand, to build muscle after any one meal, so a 16 ounce Porterhouse is going overboard by any measure). It also suggests that supplements or food products with added lecithin or carnitine would be best left on the shelf, at least until we know more.